Method and apparatus for removing substances from gases

ABSTRACT

The present invention concerns a method and an apparatus for removing substances from gases discharged from gas phase reactors. In particular, the invention provides a method for removing substances contained in gases discharged from an ALD reaction process, comprising contacting the gases with a “sacrificial” material having a high surface area kept at essentially the same conditions as those prevailing during the gas phase reaction process. The sacrificial material is thus subjected to surface reactions with the substances contained in the gases to form a reaction product on the surface of the sacrificial material and to remove the substances from the gases. The present invention diminishes the amount of waste produced in the gas phase process and reduces wear on the equipment.

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/619,820, filed Jul. 20, 2000, which claims the prioritybenefit under 35 U.S.C. §119 of prior Finnish Application No. 991628,filed Jul. 20, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the removal of substancescontained in gases, such as gases flowing at low pressure. Inparticular, the present invention concerns a method and an apparatus forremoving unreacted reactants and vapor phase precursors present in gasesremoved from vapor phase reactors.

[0004] 2. Description of the Related Art

[0005] In the atomic layer deposition method (ALD), the substrate istypically located in a reaction space, wherein it is subjected toalternately repeated surface reactions of at least two differentreactants. Commercially available technology is supplied by ASMMicrochemistry Oy, Espoo Finland under the trademark ALCVD™. Accordingto the method, the reactants are admitted repetitively and alternatelyone reactant at a time from its own source in the form of vapor-phasepulses in the reaction space. Here, the vapor-phase reactants areallowed to react with the substrate surface for the purpose of forming asolid-state thin film on the substrate, particularly for use in thesemiconductor arts.

[0006] While the method is most appropriately suited for producingso-called compound thin films, using as the reactants starting materialsor precursors that contain component elements of the desired compoundthin-film, it may also be applied to growing elemental thin films. Ofcompound films typically used in the art, reference can be made to ZnSfilms employed in electroluminescent displays, whereby such films aregrown on a glass substrate using zinc sulfide and hydrogen sulfide asthe reactants in the growth process. Of elemental thin films, referencecan be made to silicon thin films.

[0007] An ALD apparatus comprises a reaction space into which thesubstrate can be placed, and at least two reactant sources from whichthe reactants used in the thin-film growth process can be fed in theform of vapor-phase pulses into the reaction space. The sources areconnected to the reaction space via reactant inflow channels. Outflowchannels (pumping lines) are attached to a pump and connected to thereaction space for removing the gaseous reaction products of thethin-film growth process, as well as the excess reactants in vaporphase.

[0008] The waste, i.e., the non-reacted reactants removed and dischargedfrom the reaction space, is a serious problem for ALD processing. Whenit enters the pumping line and the pump, the waste gives rise to tediouscleaning and, in the worst case, the pump will rapidly be worn out.

[0009] Filtering of the gases and/or contacting of the gases withabsorbents gives some help but both methods have been shown to beunsatisfactory in the long run. Building expensive heated pumping linesin order to move the waste though the pump does not help, because theproblematic waste does not comprise superfluous amounts of separateprecursors, such as water, titanium chloride or aluminum chloride, thatcan easily be pumped as separate materials. The problem arises when thematerials are reacting, forming by-products having a lower vaporpressure, inside the pumping line. The problem is especially relevantwhen the reactants react with each other at temperatures lower than theintended process temperature, causing improper reactions. At thosetemperatures, oxychlorides might form in exhaust lines as a by-productof exemplary metal oxide deposition processes using metal chlorides asone of the ALD precursors. These by-products form a high volume powder.Typically this kind of reaction happens inside the pumping line betweenthe reaction zone and the colder parts of the pumping line. Anotherproblem occurs when precursors with a high vapor pressure at roomtemperature reach the pump sequentially at temperatures suitable forfilm growth. This might lead to a film material build-up on the surfacesof the pump. The material build-up can be very abrasive. This is aspecific problem with heated pumping lines and hot dry pumps. This willcause the filling of tight tolerances and due to that the parts willcontact each other and pump will crash. A third problem is the reactionsbetween condensed portions of the previous reaction component and thevapor of the following pulse in the pumping line. This will causeCVD-type material growth and significant powder propagation.

[0010] As mentioned above, different solutions based on filtering and/orchemical treatment of the reaction waste have been tried for decades inprocess fore-lines, with more or less poor results. Formed by-productsand powder tend to block the filters and due to the low process pressurethe gas flow is too weak to keep the mesh of the filter open. Theblocked filter will cause an additional pressure drop and thereforecause changes in the material flow from the source. Also, the processpressure and the speed of the gases will change. Attempts have been madeto use cyclones and rotating peelers to remove the by-products from themesh. By these means, some of the solid waste can be removed, but stillthe precursors with high vapor pressure will reach the pump and formby-products there.

[0011] Finnish Patent No. 84980 (Planar International Oy) discloses asystem consisting of a condensation chamber, where the gas stream isslowed down and where a big part of the waste is condensed. Beforeentering the filter unit, extra water is injected into the filterhousing to increase the by-products' particle size in order to preventblockage of the filter mesh before the waste is removed by a rotatingpeeler system. Although this apparatus represents a clear improvement ofthe state of the art, it is still not completely satisfactory.

SUMMARY OF THE INVENTION

[0012] It is an aim of the present invention to eliminate the problem ofthe prior art and to provide a simple and reliable technical solutionfor removing waste from the reaction zone of an ALD reactor.

[0013] The present invention is based on the concept of processing allof the extra precursor material of the pulse dose, to form the endproduct, before the precursors are discharged from the reactor or thereaction zone. Thereby, the volume of the waste can be greatly reduced.The postprocessing of the precursor excess stemming from the ALD processis carried out by placing a sacrificial material with a high surfacearea (typically porous) in the reaction zone, which is swept by theprecursors during their travel to the outlet of the reaction chamber.Alternatively, the material with high surface area can be placed in aseparate heated vessel, outside the reaction zone but upstream of thedischarge pump. The material with a high surface area is, however, inboth embodiments kept essentially the growth conditions (for example,same pressure and temperature) as the reaction zone to ensure growth ofa reaction product on the surface thereof. As a result, the materialwith a high surface area traps the remaining end product on its surface,thereby reducing the amount of reactant reaching the pump.

[0014] The present apparatus includes a reaction zone arrangeddownstream from (i.e., after) the reaction process, comprising amaterial with a high surface area and maintainable at essentially thesame conditions as those prevailing during the gas phase reactionprocess. The reaction zone further includes gas flow channels forfeeding gases discharged from the gas phase reaction process into thematerial with a high surface area and discharge gas channels fordischarging gas from the material with a high surface area.

[0015] Considerable advantages are obtained with the present invention.Thus, the material with a high surface area will trap on its surface theend product of the reaction of the excess gaseous reactants. The surfacearea of the trap is generally large, on an average about 10 m²/g to 1000m²/g; for example it can have the surface area on the same order as thatof a soccer stadium. The trap can be in use for several runs before itis cleaned or replaced with a new one. The pump connected to thereaction space has only to cope with materials in gaseous form becausemostly non-reactive gaseous by-products from the process reach the pump.“Non-reactive,” as used herein, refers to species other than theintended ALD reactants. The solid thin film product is substantiallycaptured in the reactant trap; this will considerably reduce wear of theequipment.

[0016] The present invention is generally applicable to any gaseousreactants. It is particularly advantageous for reactions that formcorrosive or otherwise harmful side products during the reaction of thegaseous reactants. Thus, a preferred embodiment is for dealing with thewaste generated in a vapor phase reaction using chloride-containingreactants such as aluminum chloride, which are reacted with water toproduce a metal oxide. The present invention is preferably used for ALD,but it can also be used for treating exhaust from conventional CVDprocessing or electron beam sputtering and any other gas phase processesin which the discharged gaseous reactants may react with each otherdownstream of the actual reaction zone housing the substrate. In thefollowing description, the invention will, however, be described withparticular reference to an ALD embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Next the invention will be examined in more detail with referenceto the attached drawings depicting a number of preferred embodiments.

[0018]FIGS. 1a and 1 b are schematic top plan (FIG. 1a) and sideelevational (FIG. 1b) views of a reactant trap comprising porous platesinside a suction box of an ALD reactor, constructed in accordance with afirst preferred embodiment.

[0019]FIGS. 2a and 2 b are schematic top plan (FIG. 2a) and sideelevational (FIG. 2b) views of a reactant trap, constructed inaccordance with a second preferred embodiment of the present invention,having an arrangement of plates inside a separate postreactor connectedto the suction box of an ALD reactor.

[0020]FIGS. 3a and 3 b are views corresponding to FIGS. 2a and 2 b, withthe plates replaced by glass wool cartridges, in accordance with a thirdembodiment.

[0021]FIGS. 4a and 4 b are schematic cross-sections of a cartridgefilled with glass wool (FIG. 4a) and a cartridge filled with graphitefoil (FIG. 4b).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Generally, the present invention is based on the idea ofplacing—between the substrates of an ALD reactor and the pump—a materialwith a high surface area, which forms a postreaction substrate for thedischarged superfluous gas phase reactants leaving the actual reactionzone. It is preferred that the surface of the porous material is solarge that all of the superfluous material can adsorb upon surfaces ofthe reactant trap and then be converted into the corresponding finalcompound when the next reactant pulse enters, according to the principleof ALD (Atomic Layer Deposition).

[0023] The postreaction reactant trap can be placed inside the vacuumvessel, within the hot reaction zone, or it can be formed as a separatechamber between the process chamber (or primary reaction zone) and thepump; even the space of the suction box can be used as a holder for thetrapping receptacles.

[0024] The following example relates to growing an aluminum oxide layerwith the ALD technique. In a 3,000 cycle Al₂O₃ process, 100 g of AlCl₃and 100 g of H₂O is consumed. Roughly one-third (60 g) of the reactantsends up as Al₂O₃, of which aluminum represents 30 g and oxygenrepresents 30 g. Two-thirds of the consumed reactant mass will form HClin an amount of 140 g. One-third, equaling 20 g of the precursors, isused in the thin film product grown on the substrates; the remainingtwo-thirds (40 g) of Al₂O₃ is preferably captured by the trap. Thismeans roughly 40 g of solids in the trap per run. The deposited Al₂O₃ ineach run has a thickness of 150 nm, which corresponds to a film growthof 15 μm on the trap surface after 100 runs. By selecting the pore sizeand path length so that there is essentially no pressure drop over thetrap and that any reaction products can be purged away before the nextpulse enters the trap, the thin film grown in the trap will not restrictthe gas flow.

[0025] It is particularly desirable to avoid formation of largemolecules, such as oxychlorides, that would occupy a large volume andblock the flow paths of the material with a high surface area.

[0026] According to the preferred embodiment, the sacrificial trappingblock(s) or plates can be made of any suitable material with a highsurface area, preferably porous (e.g., graphite, such as porous graphitefoils, alumina (Al₂O₃) or silica). Various ceramic materials, e.g.,honeycomb ceramics, and other mineral materials such as glass wool, canalso be used. Reticulated Vitreous Carbon is another example of asuitable material. The material should withstand the physical andchemical conditions of the reaction zone (reaction temperature andpressure; it should be chemically inert to the reactants but able toadsorb the ALD reactants). Further it should have a large surface so asto allow for a reaction of the gaseous reactants on the surface thereofin order to form the reaction product (such as aluminum oxide).Generally, the surface area of the trap material is 10 m²/g to 2000m²/g, in particular about 100 m²/g to 1500 m²/g. One alternative is tohave a porous ceramic material with a roughened surface which will allowfor penetration of the gaseous reactants into the material, leavingby-products such as hydrochloric acid, on the surface so that it can bemore easily purged away. The pores of the porous material should not betoo narrow and deep so that the (non-reacted) residues of the previouspulse cannot be purged away before the next pulse is introduced.Material having an average pore size on the order of about 10 to 100 mis preferred.

[0027] It is also preferred that the surface of the reactant trap islarge enough that the same trap material can be used for the growth ofseveral batches of thin-film elements. As discussed above, the excess ofreactant is generally 4 to 5 times the amount needed for covering thesurface of the substrates with a thin film of desired thickness.Therefore, the surface area of the material is preferably at least 4 to5 times larger than the total surface of the substrates. Morepreferably, the surface should be much larger, e.g., so as to allow foruninterrupted operation for a whole day, depending on the productioncapacity of the reactor.

[0028] There should be no substantial pressure difference over the highsurface area of the reactant trap. For this reason, the material with ahigh surface area is preferably provided with flow paths which allow forfree flow of the gases while offering the gas phase components enoughsurface for surface reactions. Various ways of achieving free flow pathsto achieve minimal pressure drops are depicted in the embodiments of thedrawings.

[0029] Turning now to the attached drawings, it will be noted that inFIGS. 1a and 1 b, the reactant trap 1 (which can also be called an“afterburner”, a “downstream reaction space” or a “secondary reactionspace”) is preferably placed below the actual reaction space 2 (or“primary reaction space”) of the ALD reactor. The reactant trapcomprises a plurality of trapping plates 3, which are placed in parallelrelationship inside the suction box 4 of the reactor. Between thetrapping plates 3 there are flow channels formed to allow for thecontinued flow of the gases to the pump (not shown). When the trappingplates are made of a suitable material with a high surface area, thereactant gases will diffuse inside the plates and deposit the reactantsdue to surface reactions similar to those reactions taking place in thereaction space above, e.g., between glass substrates and the reactantvapors.

[0030] By arranging the reactant trap immediately after or under thereaction zone, a free flow path or channel for the excess reactants caneasily be arranged. Likewise, it is simple to carry out the discharge ofthe gas from the reactant trap because it is subject to the same reducedpressure, produced by the discharge pump, as the rest of the reactor.

[0031] After each reactant pulse fed in to the reaction space and,consequently, into the reactant trap 1, the reaction space is generallypurged with an inert or inactive gas, such as nitrogen. Then asubsequent gas phase pulse is fed into the reaction space (and thenceinto the reactant trap). Thus, in the example of an ALD Al₂O₃ process,an aluminum chloride pulse is usually followed by a water vapor pulse inthe reaction space to convert the aluminum chloride into aluminum oxide.The same reaction takes place on the surface of the substrates placed inthe ALD reactor and in the reactant trap. By placing the reactant trapinside the same reaction space or reaction box as the substrates, thenecessary temperature and pressure levels for achieving an ALD (AtomicLayer Deposition) reaction on the surface of the trapping material areautomatically obtained. The reactants will form the same end product,e.g., ATO or Al₂O₃, on the surface of the trap as on the substrates.

[0032] The embodiment of FIGS. 2a and 2 b is similar to that of FIGS. 1aand 1 b, with the exception that the reactant trap 11 is placed in aseparate vessel 13 kept at the same reaction conditions as the reactor.The trapping plates 12 are arranged in a similar fashion as the platesin FIGS. 1a and 1 b, but the flow channel is arranged to provide aserpentine path. In this way, a sufficient contact time with thetrapping plates can be provided. The reactant trap vessel is attached tothe suction box of an ALD reactor with a conduit.

[0033] The embodiment of FIGS. 3a and 3 b corresponds to a combinationof the embodiment of FIGS. 1 and 2, in the sense that the trappingplates 22 are placed in a separate vessel 23, but the plates are fixedin parallel relationship with flow paths between them. The plates 22 ofthe illustrated embodiment are made of glass wool.

[0034]FIGS. 4a and 4 b show replaceable cartridges 32 made of materialwith a high surface area, such as glass wool (FIG. 4a) with flow paths33 formed in said material. Similar flow paths 35 are arranged betweenadjacent layers of a graphite foil 34 wound in a spiral fashion in FIG.4b. The layers are preferably arranged at a distance of about 0.1 mm to10 mm, preferably about 0.5 mm to 5 mm from each other.

[0035] The traps of FIGS. 4a and 4 b are preferably made of aninexpensive material, such that they can be thrown away after aneffective period of use.

[0036] In the embodiments of all of the FIGS. 2 to 4, the operation ofthe precursor trap is quite similar to that described in connection withthe embodiment of FIGS. 1a and 1 b. The material with a high surfacearea is maintained at a temperature similar to that of the actualreaction zone (i.e., depending on the precursors and the substrate,preferably about a 50° C. to 600° C., more preferably about 200° C. to500° C.). The pressure can be atmospheric, but it is generally preferredto work at reduced pressure of about 1 mbar to 100 mbar (i.e., “lowpressure”). The inactive gas used for purging preferably comprisesnitrogen or a noble gas such as argon.

[0037] Although the above embodiments have particular utility in thepreparation of thin-film structures on all kinds of surfaces forsemiconductor and flat panel devices, it should be noted that it can beapplied to any chemical gas vapor deposition reactor (e.g., CVD or ALD),including the preparation of catalysts using thin film coatings.

What is claimed is:
 1. A method for removing substances contained inexhaust gases discharged from gas phase reaction processes, comprising:carrying excess reactant from gas phase pulses of an atomic layerdeposition (ALD) process conducted upon a deposition substrate under afirst set of reaction conditions; and directing the excess reactant tocontact a sacrificial material downstream of the substrate andmaintained at substantially the first set of reaction conditions.
 2. Themethod of claim 1, wherein the sacrificial material comprises a poroussubstrate.
 3. The method of claim 2, wherein directing comprisesconducting the ALD process on the sacrificial material to leave asacrificial layer having a composition that is substantially the same asa layer formed by the ALD process upon the deposition substrate.
 4. Themethod of claim 2, wherein the porous material comprises a materialselected from the group consisting of porous graphite materials, porousceramics, alumina, silica and glass wool.
 5. The method of claim 1,wherein the excess reactant includes a halide-containing gas.
 6. Themethod of claim 5, wherein the halide-containing gas comprises achloride-containing gas.
 7. The method of claim 1, wherein the substrateand the sacrificial material are maintained within a single reactionspace.
 8. The method of claim 1, wherein the sacrificial material isplaced in a downstream reaction space housing that is connected to anupstream reaction space housing the substrate.
 9. The method of claim 1,wherein the sacrificial material has a surface area between about 10m²/g and 2000 m²/g.
 10. The method of claim 1, wherein at the first setof conditions the deposition substrate and sacrificial material are at atemperature between about 200° C. and 500° C.
 11. The method of claim10, wherein at the first set of conditions the deposition substrate andthe sacrificial material are subjected to a pressure between about 1mbar and 100 mbar.
 12. The method of claim 1, wherein the sacrificialmaterial has a surface area sufficient to react substantially all of theexcess reactant.
 13. The method of claim 1, wherein the sacrificialmaterial comprises a porous material having an average pore size ofabout 10 μm to 100 μm.
 14. An apparatus for removing substancescontained in gases discharged from a gas phase reaction process,comprising a reaction zone configured to receive exhaust flow from thereaction process, the reaction zone including a sacrificial materialmaintainable at a set of conditions substantially the same as thoseprevailing during the reaction process.
 15. The apparatus of claim 14,wherein the reaction zone includes a plurality of gas flow channels forfeeding the exhaust flow from the reaction process into the sacrificialmaterial and discharge gas channels for discharging gas from thematerial with a high surface area.
 16. The apparatus of claim 15,wherein the reaction zone further comprises baffles to define aserpentine path through the gas flow channels.
 17. The apparatus ofclaim 14, wherein the sacrificial material is porous.
 18. The apparatusof claim 17, wherein the sacrificial material has a surface area betweenabout 10 m²/g and 2000 m²/g.
 19. The apparatus of claim 18, wherein hasa surface area between about 100 m²/g and 1500 m²/g.
 20. The apparatusof claim 17, wherein the sacrificial material comprises a materialselected from the group consisting of porous graphite materials, porousceramics, alumina, silica and glass wool.
 21. The apparatus of claim 14,wherein the reaction zone includes a heating system for maintaining thesacrificial material at a temperature between about 50° C. and 600° C.22. The apparatus of claim 14, wherein the reaction zone includes aheating system for maintaining the sacrificial material at a temperaturebetween about 200° C. and 500° C.
 23. The apparatus of claim 14, whereinthe reaction zone is arranged within a common reactor shell with anupstream process region in which the reaction process is carried outupon a substrate.
 24. The apparatus of claim 14, wherein the reactionzone is arranged inside a separate reaction vessel downstream of aprocess chamber in which the reaction process is carried out upon asubstrate.
 25. The apparatus of claim 14, configured for an atomic layerdeposition (ALD) reaction process.
 26. The apparatus of claim 25,wherein the sacrificial material has a surface area sufficient to reactby the ALD reaction process substantially all excess reactant from theALD reaction process conducted upstream on at least one depositionsubstrate.